Tired eyes
Tilt aftereffect
Start the timer and stare at the + until the stripes change. Which way do the new stripes lean?
Each the opposite way to the stripes before, though they are straight up and down.
Why? The cells that sense lines tilted that way get tired, so the others win for a while.
Picture: Drawn by Science Playground, after Qcheng · Wikipedia(opens in a new tab)